At present, the aluminum electrolysis industry still employs the conventional Hall-Heroult method, electrolyte always takes cryolite-alumina as a basic system, and the currently used electrolytic cell with a prebaked anode mainly uses a carbon cathode. As a carbon cathode cannot wet molten aluminum and suffers from a long-term corrosion from cryolite, to prolong the service life of an electrolytic cell, it is usually needed to coat an inert coating on the surface of the carbon cathode. With an excellent wettability to molten aluminum and a resistance to the corrosion from cryolite, zirconium boride is highly suitable for a coating on the surface of a carbon cathode. For their expensive price, borides of transition metals such as zirconium boride are difficult to be widely applied to the coating on the surface of a carbon cathode at present.
Existing industrial zirconium boride production methods mainly include the following three types:
1: the direct reaction of the metal zirconium with the simple substance boron at a high temperature: Zr+2B=ZrB2;
2: boron carbonization method: zirconium dioxide directly reacts with boron carbide in a carbon tube in the presence of C:
2ZrO2+B4C+3C=2ZrB2+4CO, the reaction temperature is 1800-1900 degrees centigrade when the atmosphere in the carbon tube is H2 or is reduced to 1650-1750 degrees centigrade when the atmosphere in the carbon tube is a vacuum atmosphere;
3: vapor deposition method: conducting the following reaction by using ZrCl4 and BCl3 as raw materials with the participation of H2:
ZrCl4+BCl3+5H2=ZrB2+10HCl, the deposition temperature is 8000-1000 degrees centigrade, and a product of an abrasive grade and a product of an electronic grade can be obtained.
Expensive simple substance boron, the industrial yield of which is not high (usually lower than 90%) but the production cost of which is high, leads to the expensive price of zirconium boride and consequentially limits the large-scale industrial production of zirconium boride